Obesity and its complications stemming from type 2 diabetes mellitus are now leading health concerns worldwide. Research in the past decade has established a critical role for the arcuate nucleus of the hypothalamus and the nucleus tractus solitarius (NTS) in the CNS control of appetite and energy expenditure. We and others have shown that peptidergic neurons expressing proopiomelanocortin (POMC), located within both of these nuclei, are key cellular elements of the neural circuits that mediate energy homeostasis. The melanocortin peptides, including alpha, beta, and gamma-melanocyte stimulating hormones and adrenocorticotrophic hormone inhibit feeding and increase metabolic rate. Mutations in either of the neural melanocortin receptors (MC4-R and MC3-4) or POMC cause obesity in humans and rodent models, emphasizing the importance of this class of neuropeptide transmitter. In contrast, POMC derived beta-endorphin stimulates feeding and mediates the behavioral reinforcing properties of palatable food. We have recently discovered that a substantial proportion of POMC neurons also synthesize the classical, fast inhibitory neurotransmitter gamma-amino butyric acid (GABA), a finding that challenges the widely held assumption that POMC neurons would coexpress the excitatory amino acid transmitter glutamate. GABA was previously co-localized in an adjacent population of orexigenic, neuropeptide Y/agouti gene-related transcript neurons in the arcuate nucleus, which generally oppose the function of POMC neurons. Based on these data and other studies showing qualitative differences among POMC neurons in their individual responses to hormonal and synaptic signals, we hypothesize that POMC neurons are not a synchronized population of homogenous cells, but instead composed of subpopulations with distinct roles in energy balance. Furthermore, we postulate that the coordinated release of GABA and peptide modulators from POMC neurons may be a key aspect of their mechanism of action at autoreceptors, and pre- and postsynaptic sites on other neurons. Therefore, the overall goal of this project is to further delineate the neurochemical basis of subpopulations of POMC neurons within the arcuate nucleus and NTS and to match these subpopulations with their specific actions in the regulation of metabolic rate and food intake. The specific aims are to: 1) Utilize a newly developed mutant mouse model with selective neuronal deficiency of POMC to determine the specific physiological functions of the different melanocortin receptor ligands in the central nervous system control of energy homeostasis; 2) Map the co-localization of GABA and POMC peptides within the arcuate nucleus and hypothalamic projections of POMC neurons and test how GABA release from POMC neurons is integrated with the release and physiological activity of melanocortins and beta-endorphin on energy homeostasis; and 3) Probe the functional organization of the arcuate nucleus and NTS using the genetic techniques of: a) Cell-specific ablation of selected groups of POMC neurons by transgene-directed toxin delivery; and b) Aggregation chimera analysis to juxtapose multiple different combinations of wild-type and POMC-deficient neurons within the brain. Results from these studies are expected to significantly increase our basic understanding of the brain areas and mechanisms involved in the regulation of body weight and lead to new therapeutic approaches for the prevention and treatment of obesity.